This invention relates to methods of forming an exoergic structure or coating from reactive exoergic materials and to exoergic structures and coatings produced by such methods. This invention further relates to the formation of said exoergic structures and coatings in arbitrary shapes by plasma spraying upon a suitable substrate. Still further, this invention relates to the plasma spraying of reactants, which are capable of sustaining a combustion synthesis reaction, into exoergic structures or coatings and igniting said plasma sprayed reactants either under pressure or not under pressure to form refractory materials of varying densities.
Exoergic materials and structures produced therefrom have long been known in the art. As used herein, "exoergic materials" are materials which react to release relatively large quantities of energy. Such materials contain all of the components necessary to sustain an exothermic reaction, in and of themselves. Thus, such materials can contain an oxidizing agent and an agent to be oxidized. Such materials can be ignited by suitable means to produce a conventional, self-propagating exothermic reaction and have applications as "one-shot" chemical heat sources.
Other exoergic materials, if the amount of exothermic heat released is sufficient, can be ignited in a nonconventional combustion mode known as "self-propagatory high temperature synthesis" (SHS) or combustion synthesis. Preferably, such a combustion synthesis is carried out under pressure and results in a useful product, preferably a dense refractory material.
The amount of exothermic heat released depends upon the particular chemical system. For example, the heat of formation of silicon carbide (SiC) from silicon and carbon powders is 300 cal/g; whereas the heat of formation for titanium diboride (TiB.sub.2) from titanium and boron powders is 1200 cal/g. When the reaction has sufficient chemical energy to be carried out by combustion synthesis, the process is characterized by a rapidly moving combustion front and self-generated high temperatures in the product phase.
Exoergic materials have been formed into shapes, for instance, by pressing into pellets, rings, rods or cups for use in welding applications and the like. However, many useful configurations cannot be readily achieved by pressure-forming powder mixtures because of die-shape restrictions. For example, thin-walled shapes cannot be pressed because large length-to-diameter ratios result in nonuniform densities. Accordingly, known methods of producing exoergic structures are deficient, and there has been a continuing need for improvement.
Deposition of powdered materials onto substrates by the use of plasma guns has been known for many years. Exemplary patents are U.S. Pat. Nos.3,387,110; 3,591,759; 3,676,638; 4,121,083; and 4,146,654. It is to be noted that many of these patents relate to plasma flame-spraying which is entirely nonanalogous to plasma spraying per se.
Although such plasma spraying techniques exist, conventional wisdom in the art dictates that such techniques cannot be used in the formation of exoergic structures since exoergic materials would be expected to react violently, releasing large quantities of energy, in the plasma. In one aspect, this invention thus relates to methods of forming exoergic structures and coatings by plasma spraying exoergic materials without chemically reacting said materials.
This application further relates to a method of producing dense refractory materials by combustion synthesizing the plasma sprayed appropriate exoergic materials capable of sustaining a combustion synthesis reaction. As indicated above, materials having sufficiently high heats of formation can be synthesized in a combustion synthesis process characterized by a combustion wave which, upon ignition, spontaneously propagates throughout the reactants, converting them into products.
The use of a combustion reaction to synthesize a refractory material was first considered by Walton et al. [J. Am. Ceram. Soc., 42(1): 40-49 (1959)] who produced a composite ceramic/metallic material using thermite reactions. In the late 1960's, A. G. Merzhanov and his colleagues began work on self-propagating combustion reactions which led to the development of a process which they called "self-propagating high temperature synthesis" (SHS). [See Merzhanov et al., Dokl. Chem., 204 (2): 429-32 (1972): Crider, Ceram. Eng. Sci. Proc., 3 (9-10): 538-554 (1982).]
Self-propagating high temperature synthesis (SHS), alternatively and more simply termed combustion synthesis, is an efficient and economical process of producing refractory materials. [See for general background on combustion synthesis reactions: Holt, MRS Bulletin, pp. 60-64 (Oct. 1/Nov. 15, 1987); and Munir, Am. Ceram. Bulletin, 67 (2): 342-349 (Feb. 1988).] The combustion reaction is initiated by either heating a small region of the starting materials to ignition temperature whereupon the combustion wave advances throughout the materials, or by bringing the entire compact of starting materials up to the ignition temperature whereupon combustion occurs simultaneously throughout the sample in a thermal explosion.
In the synthesis of refractory materials by conventional methods, the chemical reaction is initiated and carried to completion by heat from an external source such as a furnace. Usually, the heating rate is purposely kept low to avoid large temperature excursions caused by the high heats of reaction. Refractory materials prepared by such conventional methods are relatively expensive due to the high cost of energy and equipment. In the combustion synthesis process, however, after ignition has occurred, the rest of the sample is subsequently heated by the heat liberated by the reaction without the input of further energy. As a result, the power needed is much lower, and expensive equipment, such as high temperature furnaces, is not required.
Advantages of combustion synthesis include: (1) higher purity of products; (2) low energy requirements; and (3) relative simplicity of the process. [Munir, supra at 342.] However, one of the major problems of combustion synthesis is that the products are "generally porous, with a sponge-like appearance." [Yamada et al., Am. Ceram. Soc., 64 (2): 319-321 at 319 (Feb. 1985).] The porosity is caused by three basic factors: (1) the molar volume change inherent in the combustion synthesis reaction; (2) the porosity present in the unreacted sample; and (3) adsorbed gases which are present on the reactant powders.
Because of the porosity of the products of combustion synthesis, the majority of the typical materials produced are powders or porous (40-60%) compacts. If dense materials are desired, the powders or compacts then must undergo some type of densification process, such as sintering or hot pressing. The ideal production process for producing dense SHS materials would combine the synthesis and densification steps into a one-step process.
The present invention solves the problem of porosity of combustion synthesis products by plasma spraying the reactant powders onto a suitable substrate to form a fully dense, unreacted body or coating. Because the plasma spraying is a high temperature operation, any absorbed gases are removed in the hot plasma. The resulting structure comprises a fully dense, homogeneous mixture of the reactants. Rapid quenching of the powders upon impact, noncontact in the plasma flame, and short dwell times in the plasma, prevent the occurrence of a chemical reaction between or among the reactants. Both reactants melt in the plasma prior to deposition. Therefore, this aspect of the invention obviates two sources of porosity as described above for combustion synthesis.
The plasma-formed body can take on various near net and net shapes from both simple to complex, ranging from flat or cylindrical compacts to hemispheres, spheres or other more complicated shapes created by the design of a mandrel or mold. As indicated above the plasma-formed materials may be monolithic structures or single or multi-layered coatings.
The plasma-sprayed fully dense reactant materials are then ignited preferably under an inert gas pressure whereupon a combustion wave rapidly sweeps through the materials transforming them into the product phase. This inventive process differs from conventional (non-SHS) means of producing dense refractory materials of various shapes, such as hot-isostatic-pressing for the following reasons, among others: (1) the synthesis of the product occurs during combustion; (2) there is no need for a high temperature furnace since the chemical energy released in the process supplies the heat that is necessary for densifying the product; and (3) a barrier material, such as a metal canister or glass envelope, is not required since the reactant materials are fully dense and self-contained.
Other methods of densifying combustion synthesis products have been used and include the following approaches: (1) the simultaneous synthesis and sintering of the product; (2) the application of pressure during (or shortly after) the passage of the combustion front; and (3) the use of a liquid phase in the combustion process to promote the formation of dense bodies. [Munir, Am. Ceram. Bulletin, 67(2): 342-349 at 347 (Feb. 1988).] However, a unique and salient aspect of this invention is that not only do the methods result in dense refractory materials, but also the materials are created in the shape required for the intended use. This feature is especially important in that dense refractory materials are very hard (for example, TiB.sub.2 has a hardness comparable to diamond) and are therefore difficult to grind to the requisite shape. The methods of this invention result in refractory materials of near net or net shape.
The combustion synthesis of plasma-formed materials not only minimizes the porosity of the products, but also promotes their uniformity. Other advantages of said processes will be evident from the examples, claims and description below.